VSR FPS drop with higher power spring?

[Tiercel]

So I got a lovely little VSR that was firing bang on 345fps consistently with .2s, obviously MORE POWER needed, so I put a Laylax 170SP spring in it and it fires at a whopping... 280fps with .2s (and reads at 190fps on .4s). Now obviously this isnt right, so I put the original spring back in, and its still firing at 280fps. This confuses me greatly. The obvious answer I suppose is air leak somewhere or Ive put something back together wrong. However I cant figure out for the life of me whats missing or causing an air leak, everything seems to be right to me. The piston & cylinder are stock, the only thing Ive noticed is the big thick rubber washer on the piston head looks warped and perished, however I dont think that would be causing such a huge FPS drop, especially if it was fine before I took it apart. Unless the act of taking it apart has finished off the dying washer totally now. Yes the spring is the right way round. 

Im also partly convinced my chrono isnt reading it properly (Xcortech 3200x) as it keeps reading TOUT rather than an FPS, and when it does pick up an FPS its very low. It only reads about 1 in 10 shots, the rest will be TOUT. Testing it with a known gun that fires at 330fps, it still reads 330fps for that and picks it up every time no problem. It just doesnt seem to like reading the VSR. Im wondering if perhaps its travelling too fast for it to get a proper reading hence all the TOUT readings. Im not sure what its rated to read up to, apparently it can be used for air rifles so it shouldnt have a problem reading a 500fps+ BB Im sure.

To me the BBs dont look like theyre only flying at 280fps using the Mk1 human eyeball chrono theyre flying far and straight, they can launch a .4 into the sky with only a tiny bit of hop and it flies... well too far for me to measure it, its about 50 meters to the end of my communal garden and it dissapears into the treetops behind that, so 60+ meters before it even starts to drop off. I cant imagine it doing that if its only putting out < 280fps 

I dont know the brand or history of the VSR, bought it second hand on here for a bargain price with the assumption it would be rubbish and I could use it as a project gun. It actually shoots very well and the previous owner bought it from someone else who has put some unknown upgrades in it. Its got a few airsoft pro branded bits in the hop unit, an unknown aftermarket trigger unit, and an unknown tightbore barrel. Which doesnt help much when trying to figure out whats going wrong or what needs changing.

 

[proffrink]

Before you waste any time on disassembly and diagnosis, get a second cronograph's opinion.

 

[Tiercel]

Sadly nobody else I know has one, so my only choice for that is take it to a site and hope for the best! I can't figure why if it's the chrono that it works fine with every other gun I have.

 

[proffrink]

Do you have any guns that shoot as hot as it though?

 

[Tiercel]

Nope. Only pistols and regular AEGs all around the 280-330fps mark

 

[proffrink]

Yeh, cronographs will spit out weird numbers when they're having issues. I'd really just get that second opinion first - even if it requires taking your gun to the next site you attend (assuming you don't desperately need it). There have been a fair few in the past on here who have had problems with the crono readings at higher muzzle energies, and I'd hate to think that you rebuild the whole thing for nowt.

If it does turn out to be the spring doing this, I'd check the hop unit first - be sure your bucking is well seated and PTFE'd to the barrel so it's not getting blown out. Ensure the cylinder head isn't dinged up and is letting air out.

Of course all things equal this problem would be obvious with your old spring too. The only thing I could think of that would make it worse with a higher power spring would maybe be the bucking getting blown out when more air is forced down that barrel, but those are slim odds.

 

[Nick G]

Try dry firing with a thumb over the end of the inner barrel. Does the piston slam fully home ? if so you've got an air leak. If it doesn't slam home until you take your thumb away you are leak free. Since you've had the bolt open to replace the spring this is probably the most likely culprit. Ideally do the thumb test with your thumb over the cylinder nozzle , although that will require removing the barrel etc. Try re assembling the bolt with PTFE tape on the cylinder head threads to see if that helps . It could be hop unit related , but given the only thing you changed was the spring , it seems likely its related to that.....assuming your chrono is behaving.

 

[Tiercel]

Try dry firing with a thumb over the end of the inner barrel. Does the piston slam fully home ? if so you've got an air leak. If it doesn't slam home until you take your thumb away you are leak free. Since you've had the bolt open to replace the spring this is probably the most likely culprit. Ideally do the thumb test with your thumb over the cylinder nozzle , although that will require removing the barrel etc. Try re assembling the bolt with PTFE tape on the cylinder head threads to see if that helps . It could be hop unit related , but given the only thing you changed was the spring , it seems likely its related to that.....assuming your chrono is behaving.

Gave this a try (took a few days for a day off work to tinker with guns!). After I took it apart the first time, the nozzle threads were already PTFEd so I made sure to redo it when reassembling. Took it apart again today and re-did the PTFE and put a thin dab of silicone grease on the O rings to make sure theyre not dried out. So with my thumb over the cylinder nozzle and firing it out of the gun, the piston fires forward about halfway then stops, and very slowly creeps forward like a mm or two per second until I take my thumb away and it slams forward. So I think the air seal is good. I did take apart the hop when I first got it too to a) inspect whats inside and b) clean it out (inner barrel and hop rubber were disgustingly dirty) with a few dabs of rubbing alcohol on a cotton wool bud. Ive not tried the dental floss/PTFE mod on the hop rubber but seeing as its in a fully enclosed metal hop chamber with no real room for it to expand or blow off I dont think thats the case, however Ill try a thin wrap of PTFE around the end of it and see what happens.

 

[Nick G]

Its worth doing the floss trick. The hop chamber on my Well L96 is also fully enclosed and tight fitting but I was still losing air until I flossed it. Now I can fire the gun with my thumb over the end of the inner barrel and have the piston stall part way. Easy to do so got to be worth a go. Sounds like the piston is sealing pretty well so it can only really be the hop.

To give you an idea , I am running an Action Army bucking in the standard hop chamber and the standard barrel. I have down sprung it form stock and I think the spring in there is a 150. The gun chronod on site yesterday using .20's at 500, 499 and 499 . The hop is set for .43's because I happen to have some. I would normally run .36 or .40 because its got quite a long barrel. I've got a 6.03 barrel to fit during the week and I expect I will have to down spring it again.

It can only be an air leak or a restriction in barrel, probably from the hop. Unless its your chrono which is certainly possible especially as it sounds like the gun is firing well

 

[Tiercel]

PTFE taped the hop rubber, and its still firing at 250fps. ? though no more TOUT, chrono seems to be picking it up reliably every time now.  One thing I did notice was this:

the opening at the firing end of the outer barrel (barrel plug I guess for lack of a better name?) is very very tight and has an unusual hexagonal shape rather than round, this looks like its virtually touching the walls of the inner barrel, so I removed it, and the chrono readings jumped to 280-290 fps. Im wondering if BBs are perhaps lightly grazing this and slowing down... but then Id expect them to all be random flyers rather than shoot so well. Im thinking of drilling this out into a wider, 8-10mm round hole now anyway.

Ive also noticed that the 170SP spring is a good 3-4cm longer than the stock spring, though looking at the stock spring it does look like its had a home made cut down job done on it, albiet quite a good one where its been ground flat but obviously not factory made. Perhaps the spring just doesnt fit or compress enough to get the full FPS? 

At this point Im mostly convinced it has to be my chrono, so Im just gonna take it to the next game and see what happens. Im open to other suggestions to try in the meantime. 

 

[Nick G]

Sounds like a good plan, once you know how the site chrono compares to yours it will help narrow things down. Hows the compression with the ptfe on the hop rubber, does the piston stall with your thumb over the inner barrel ?

 

[Tiercel]

So taking the end off the outer barrel and wedging my  thumb over the inner barrel, the piston fires as normal it doesnt stall at all unlike when I do it over the cylinder head. I still dont think I can be losing 2-300 fps on an air leak surely?

 

[Nick G]

Maybe not losing 2-300 but you are certainly loosing plenty. Interestingly I fitted the 6.03 barrel to my L96 today and the bucking leaked like a sieve. I had to tape it and floss it to get a decent seal. Same thing as you , piston fired as normal with thumb over barrel and fps well down in the low 400's After going to town on the seal I can stall the piston again and am back at high 490's

It may be a combination of air leak from the bucking and that barrel end cap if the BBs were hitting it. I would sort those two issues and then see where you are at, there could still be an issue with your chrono and possibly the spring from what you were saying earlier. Can you borrow a known spring to try ?

 

[Tiercel]

So I got a second opinion chrono reading at the NAE, its coming up around 390-ish FPS. Still not great considering the spring in it. However it performed admirably and pinged many Deltas who thought they were being sneaky sitting in a bush or well out of range So its definitely a keeper! Time to lavish money upon it for some nice upgrades. I think next step is going to be a replacement hop unit, either the Airsoft Pro or Action Army ones both seem to get equally good reviews and many happy users saying theyre practically airleak proof due to being such tight construction. A new hop rubber wouldnt hurt too incase this one is old and worn and potentially responsible for airleak.